1. Field
This application relates to tubular membrane modules, specifically to the method of manufacturing such membrane modules, and even more specifically to the physical means of sealing the ends of the tubes to make suitable contactors with isolated or separated lumens.
2. Background of the Invention
Membrane contactors are useful devices for separation processes, contacting processes, or as filters. A membrane contactor consists of a membrane or membranes held in such a manner as to separate two regions of flow and enable the membrane to act as a separation means between the two phases, and a housing to enclose the membrane and contain and direct the flow of the multiple phases. The membrane acts as a barrier between the two fluid phases and selectively allows or prohibits the transport of one or more chemical species or particles from one fluid stream to the other. The housing has one or more ports to allow flow to and from the membrane. Membrane contactors can be considered as a subclass of the more general class of fluid or fluid/gas transport devices.
Membrane contactors have applications as filters, separation systems, or contacting devices in many industries such as chemical, pharmaceutical, food and beverage, environmental, water treatment, and semiconductor processing. Membrane separation processes such as gas/liquid separation or membrane distillation are replacing their bulk counterparts (distillation towers, stripping columns) due to improved energy efficiency, scalability, the ability to operate isothermally, and smaller physical footprints. In addition, membrane filters, separators, and contactors generally have no moving parts and are physically simple and rugged, resulting in low maintenance cost.
A membrane filter is a structure or physical device that employs a membrane to create a physical barrier that separates two phases and restricts the transport of particles, gels, bacteria, or viruses in a selective manner from one phase to another.
A membrane separator or contactor is a structure or physical device that employs a membrane to create a physical barrier, again separating two phases and allowing selective transport of one or more chemical species from one phase to the other. A membrane separator is a device in which one species is selectively removed from a fluid across a membrane barrier. A membrane contactor is a device in which one or more species are introduced into a fluid across a membrane barrier.
Hollow fiber and tubular membrane devices are a broad class of membrane modules that employ membranes in hollow fiber or tubular form. In general terms, either geometry consists of a membrane with a generally cylindrical shape, having an outer diameter, a hollow channel (also known as a lumen) opened axially down the center of the cylinder parallel to the major axis of the cylinder defining an inner diameter, ensuring a uniform membrane wall thickness (defined as the difference between the outer diameter and the inner diameter divided by 2) when measured both circumferentially and axially, with the hollow channel open on at least one of the two ends of the cylindrical membrane.
Hollow fibers and tubes are geometrically similar and the distinction between a hollow fiber and a tube is made in terms of the diameter of the hollow fiber or tube. The distinction is not a sharp one, and for the sake of this patent application we delineate a tube opposed to a fiber when the diameter of the hollow fiber approaches 2 millimeters and greater. The advantages of the two over other membrane geometries, specifically over membranes in sheet form, are similar.
While improved surface to volume ratio favors the use of smaller diameter hollow fibers, certain processes dictate the need for larger diameter tubes. Filtration systems that have high solids in the inlet streams require larger diameter tubular membranes to avoid becoming plugged. Generally these filtration systems are run in a cross flow manner, maintaining high velocities of the product to be filtered moving through the lumen of the membrane. The high velocity helps eliminate the build-up of a cake or solids on the membrane wall and thus shutting down the filtration process. High solids cross flow filtration applications are very common in the food and beverage industry, waste water systems, and other industrially important filtration processes.
While many types of membranes are available in sheet form, the ability to create significantly higher surface area per unit volume with a hollow fiber or tubular membrane is of major advantage to the designer and user of a membrane filter or contactor. A hollow fiber or tubular membrane is also typically self-supporting in contrast to flat sheet or thin film membranes that usually require a skeletal structure for support. In addition, typical contactor designs employing hollow fiber or tubular membranes, whether constructed as a cross flow element or in a dead-end configuration, offer more uniform flow and fewer regions for the flow to stagnate. In this patent application tubular membranes are employed, and typically a porous tubular membrane is employed, however the invention is not limited to porous membranes.
The usefulness and efficiency of a membrane contactor is determined by the available surface area of the membrane per unit volume of the device and the rate at which the transfer or removal of the species of interest occurs; this is generally governed by the flux (flow per unit area, per unit time, per unit pressure gradient) of the process stream. The available surface area for a hollow fiber membrane module is dictated by the packing density of the fibers (the ratio of the sum of the cross sections of the individual fibers to the total available cross sectional area). The higher the packing density and the greater the surface area to volume ratio generally results in a more efficient module.
Two other useful parameters for defining the performance of a porous membrane are the pore size distribution and the porosity. The pore size distribution is a statistical distribution of the range of pore diameters found in the membrane wall. The smaller the mean pore size, the smaller the particle a membrane filter will separate. The largest pore size can also be characterized by a measurement called a bubble point, which is defined as the pressure at which the first air bubble is observed escaping through a membrane wall on a fully wetted fiber. To perform a bubble point measurement, the fiber is wetted and flushed with isopropyl alcohol (IPA) to ensure that all the pores are filled with liquid and that there is no trapped air in the pores of the fiber. The fiber is then looped and immersed in a clear container of IPA with the two lumen ends above the level of the IPA. Air pressure is applied to the lumen ends in small increments until the first bubble of air is observed on the outside of the fibers. The resulting pressure is the bubble point pressure and is an indication of the largest pore in the fiber as the IPA in that pore is the most readily (lowest pressure) displaced by the incoming air pressure.
The porosity of a hollow fiber membrane may be defined as the percentage of free volume in the membrane, or for PTFE hollow fiber membranes, as (1−membrane density/2.15)×100 where 2.15 is the density of solid PTFE. The higher the porosity, the more free volume and generally the higher the flux rate through the membrane wall.
For a given pore size distribution, higher porosities are often desirable as they lead to higher flux rates. Unfortunately higher porosities also generally lead to softer membrane walls, causing the tubular membranes to be structurally very soft and prone to deformation and collapse, especially during any assembly process.
The elements of a tubular membrane contactor are the tubular membrane itself, the housing, and a means to secure the tubular membranes to a support structure at least one end of the housing and to the housing itself. A tubular membrane, the primary element of a tubular membrane contactor is a porous or non-porous, semi-permeable membrane of defined inner diameter, defined outer diameter, length and pore size, and generally of a very high aspect ratio, defined as the ratio of the length to the diameter of the fiber. A tubular membrane contactor is generally comprised of a plurality of tubular membranes assembled with at least one common feed to the open lumen ends isolated from a common discharge from the outside surfaces of the tubular membranes. There may be a common discharge for the lumens of the opposite ends of the tubular membranes isolated in a similar manner from the outside surfaces of the membranes.
The housing is an outer shell surrounding the membrane that secures and contains an assemblage of tubular membranes. The housing is equipped with one or more inlets and one or more outlets, such that the potted bundle of hollow fiber membrane acts as a barrier and separates the two phases or process streams. The design of the housing, and specifically the relationship of the inlets and outlets, regulates the flow of the process fluid into or out of the fiber lumens and directs the processed fluid away from the device. There are typically two common modes of designing the housing, which relate to how the fluids interact with the membrane. What are known to those well versed in the art as dead-end elements consist of a housing that directs all of the volume of one fluid to pass through the membrane walls to reach the discharge or exit of the housing. The dead-end design is a very common design employed for membrane filtration. For dead-end tubular membrane filters, both ends of each tubular membrane are bound at one end of the housing. In dead-end tubular membrane filters the process fluid either enters the lumens of the tubular membrane and discharges out through the walls of the tubular membrane, or enters through the walls and discharges out of the lumens. In either case, this ensures that the entire process stream passes through the membrane wall.
A dead-end tubular membrane filter configuration is contrasted to a cross flow configuration in which the lumens are open at both ends, and only a portion of the process stream entering the upstream lumens passes through the membrane wall, while the remainder of the fluid discharges through the downstream lumen openings. The portion of the fluid discharging from the downstream lumen end may be passed along to another membrane element, recycled to the beginning of the unit, or discarded. The cross flow configuration mode is employed with both filtration as well as membrane contacting or separation processes.
A tubular membrane bundle may be integral to the housing or may be designed so that the membrane bundle may be installed and removed. The composition of the cylindrical containment shell can be perfluorinated homopolymers of PTFE, fluorinated homopolymers of PVDF (polyvinylidene fluoride), perfluorinated co-polymers of TFE/HFP, TFE/PPVE, TFE/CTFE, TFE/Alkoxy, and partially fluorinated co-polymers of Ethylene/TFE, Ethylene/FEP, and similar materials, or other chemically resistant resins such as PVC, Polysulfone, Polyethersulfone, Polycarbonate, PEEK, PEK, Polyamides, or Polyimides. Or, the cylindrical containment shell can be composed of stainless steel, carbon steel, other polymeric materials, or organic and inorganic composites.
Membranes for contactors or filters have been developed from a variety of synthetic polymers and ceramics and have been known in the industry for many years. While ceramic membranes offer the chemical resistance and high service temperature required by aggressive acidic, alkali, or organic solvent applications, in their present-day state they are very fragile, very expensive, and very difficult to work with, a combination of features that keeps ceramic membranes out of many applications.
The vast majority of state of the art polymeric membranes are limited as they are not inert, they possess inadequate chemical purity, thermal stability and chemical resistance, and occasionally have undesirable surface properties, preventing their use in certain important applications. This is because these very same membranes are spun from solution, and the fact that they must be soluble in certain solvents to convert to a membrane means that the final membrane itself is susceptible to attack by those same classes of solvents.
It has long been desired to be able to have membranes manufactured from fluorinated or perfluorinated resins due to their high service temperatures, chemical stability, inertness, and chemical resistance to a wide range of solvents, acids and alkali systems. However, membranes produced from non-fully fluorinated polymers still require aggressive solvent systems and very high processing temperatures to manufacture, increasing cost and generating environmental and waste issues. Membranes manufactured from Polytetrafluoroethylene (hereafter referred to as PTFE) are most desirable because, as a fully fluorinated polymer (with no C—H bonds on the polymer chain backbone), they offer the best combination of thermal and chemical stability of all the fluorinated and perfluorinated resins commercially available. In addition, the method by which they are converted to membranes does not employ hazardous solvent systems; instead using a stretching and orientation method.
It is also desirable to have membranes manufactured from fluorinated or perfluorinated resins, especially fully fluorinated resins, due to their low surface energy. Filtration of organic liquids, separating organic from aqueous systems, or removing vapor from aqueous systems all favor low energy membranes. PTFE offers the lowest surface energy of all the fluorinated or perfluorinated polymeric membranes with a surface energy of less than about 20 dyne-cm.
The membrane material discussed in this patent application, PTFE, a member of the fluoropolymer family, offers significant advantages over non-fluoropolymeric synthetic resin membranes. PTFE possesses extraordinarily high service temperatures. PTFE is chemically clean and inert and resistant to attack by acids, alkalis, and a very wide range of organic chemicals and solvents, and can be fashioned into a very physically strong membrane either as a flat sheet or hollow fiber or tube. Commercial interest in PTFE membranes runs high due to the above stated properties along with its hydrophobic surface, making it ideal for certain isolation operations. PTFE membranes also offer the best thermal stability and chemical resistance in the general class of fluoropolymers, making them the ideal choice for a membrane material. Furthermore, PFTE is a soft material with high compressibility. This allows for the novel use of PFTE as a self-sealing gasket as will become more apparent in this patent application.
It is widely known in the field of membrane construction that a major challenge with the use of hollow tube membranes lies in obtaining a robust seal around each tube when assembling the tubes into a contactor or module. The material used to create the seal between the tubes must: bind the tubes, seal and isolate the lumen-side face of the tubes from the downstream tube surface, and prevent the fluid being filtered or contacted from bypassing the membrane surface. The process by which the sealing material is introduced to the tube bundle is critical, as significant force or pressure will damage, collapse, or crush the individual hollow tubes, rendering the module far less effective or useless. PTFE as a membrane poses additional difficulties trying to affix it in a leak proof manner to a surface as the PTFE does not melt, and because of its high surface energy very few materials will adhere to its surface, making it difficult to glue or bond into place.
For small diameter hollow fibers, a process known as potting is often employed. Potting the hollow fiber membranes may occur prior to, or during the operation of mounting the hollow fiber membranes into the housing. To bind the ends of the hollow fibers to one another, a potting compound is employed. A potting compound is a material that when applied around the ends of hollow fibers, bonds them together into a solid, cohesive mass that isolates and fixes the hollow fibers from the remainder of the bundled assembly of fibers.
Traditional potting techniques fail with larger diameter tubular membranes for many reasons. The lower packing density resulting from larger diameter tubes leaves significant interstitial voids between the tubes that are extremely difficult to fill with a potting system. Even if the voids are successfully filled, it creates a weak point due to the different physical properties between the tubular membrane and the potting compound. The larger diameter tubular membranes would flex under pressure, pulling away from and loosening the potting compound in these large regions. Unless the potting compound creates a very strong bond with the wall of the tubular membrane, the soft nature of the membrane allows the wall to be pushed in and away from the bulk of the potting compound, generating a point of failure.
Larger tubes are harder to melt or soften, which facilitates bonding tubes to one another; this eliminates many of the fusion techniques identified in the literature for smaller tubes. Because the tubular membranes are larger diameter and soft, they tend to deform under any applied pressure during assembly. Along with these general limitations for any larger diameter polymeric tubular membrane, there are significant challenges in working with fluoropolymeric tubular membranes in general and PTFE specifically. It is nearly impossible to glue PTFE tubes to other surfaces, and if successful, due to the larger diameter, the tubes will easily pull away from the bonded surface. The larger the diameter tube, the more likely that the soft wall will be able to be stripped from the bonding surface due to the decreased surface area/cross sectional area ratio with larger diameter tubes.
In addition, potted bundles of larger tubes have structural weaknesses (which increase as the hollow fibers increase in diameter) due to increased void volume in the potted ends. This leads to additional cost and processing issues. Consequently, an alternative and effective means to isolate or mount large tubes in a contactor has been a long time endeavor in the industry.
Typically, contactor designs for tubular membranes have employed some sort of tube sheet design, similar to that employed for assembling heat exchangers. In a tube sheet assembly, the ends of the individual tubes are pulled or pushed through a series of holes drilled into a flat sheet or plate. Once the end of each tube is placed in the tube sheet, the ends of the tubes are sealed in place. For heat exchangers, where both the tubes and tube sheets are typically metal, these ends may be flared, welded, soldered, or crimped into place. Compression fittings that rely on the rigidity of the tube are often employed as well.
For rigid plastic tubes, some of these same techniques are known. Certain plastics may be welded or bonded via adhesive to a tube sheet to create a leak proof assembly. Polymeric membranes, recognized herein, tend to be softer materials and would not be described as rigid, and thus would not be suitable for crimping or compression methods mentioned above. Any compressive force on the outside of the membrane tube would cause it to crush.
Some manufacturers have had to resort to mounting the tubular membrane on a support structure or skeleton to facilitate mounting the membrane and sealing against the tube sheet. This practice is time consuming and very expensive.
It has been stated herein that there is a strong need for membranes produced from fluoropolymers due to their high service temperature, outstanding chemical resistance, hydrophobicity and other desirable properties. It has also been established that these same desirable physical properties from the standpoint of membrane properties also render the fluoropolymer membrane extremely difficult to glue or bond to other materials. This inability to easily glue or bond combined with the softness of the membrane creates a difficult problem when assembling or mounting tubular membranes into a contactor or filter.
It is the object of this invention to overcome the stated limitations for fluoropolymeric tubular membranes in general and PTFE tubular membranes in particular and provide a method for rapid and economic assembly of tubular membranes into contactor and filter modules. The method of this invention offers the following important advantages to the tubular membrane contactor designer: Rapid and economic assembly and adaptable to very large contactors; Reversible to remove damaged tubular membrane(s); Leak proof under a variety of temperatures and pressures; No voids, or dead space where flow can stagnate or debris can accumulate; No interruption in the contour of the inside wall of the membrane as it passes into the tube sheet.
The posited challenges and commercial demands of fixing and isolating tubular membranes have been solved in this patent application. We have invented a physical technique, or method, to reliably and rapidly seal and isolate the soft hollow tubes in a tube sheet with the hollow tubes acting as self-sealing tight fit gaskets. The presented self sealing method herein overcomes all the challenges listed above by not requiring the tube wall to be heated or softened and by not utilizing any additional materials that could contaminate or diminish the chemical resistance of the fluoropolymer hollow tubes. This is accomplished by a physical means of an interference fit locking the hollow tubes into a tube sheet by the insertion of a hard hollow mandrel at the end of the hollow tubes (after insertion and pulling the hollow tubes into the tube sheet). In this patent application, the hollow tubes perform as the actual sealing material as they act as self-sealing gaskets due to the compressibility of the PTFE.